The size of silica-based nanoparticles affects their physical, chemical, electrical and optical properties. Traditionally, micrometer sized silica particles have been used as catalyst substrates, pigments, stationary phase in chromatography columns, etc. Recently, nanometer sized silica nanoparticles have developed rapidly and become an important class of nanomaterial. By themselves, the size-dependent properties of pure, nanoscale silica nanoparticles are not remarkable. However, when these nanoparticles are combined with various functional molecules, the impact of size becomes significant—especially for analysis applications. The role of silica nanoparticles in these cases is usually as a supporting or entrapping matrix. Two major reasons make silica a useful matrix in this regard and particularly relevant in bioapplications. First, the surface of silica nanoparticles is easily modified based on well-established chemistry. With appropriate surface and internal functionality, these nanoparticles can be linked to a variety of biorecognition agents in many different ways (e.g., antibodies, protein complexes, nucleic acids, etc.). Second, the negatively-charged silica matrix itself provides numerous electrostatic binding sites to physically dope (i.e., adsorb) a wide variety of positively-charged molecules. When doped with bioactive molecules and medicines, these nanoparticles can serve as drug delivery vehicles capable of controlling the quantity and time of release. When doped with dye molecules, these nanoparticles become intensely luminescent reagents capable of sensitively signaling biological targets.
Variations in the size of luminescent silica nanoparticles are needed for effective imaging of a wide variety of biological samples and processes. In cell imaging, the recognition reaction occurs either on the cell surface or inside the membrane. When a nanoparticle is used as a tag, the size of the nanoparticles may affect whether it can reach a target site or not, such as a cell nucleus, for example. Furthermore, the size of a luminescent silica nanoparticle primarily determines its luminescence intensity at the optimal concentration of dye molecules. To enhance the detection levels to meet the required sensitivity of a measurement, various sizes of luminescent nanoparticles are essential.
The size-dependent properties of nanoparticles are varied but can be tied to three mainly beneficial effects. In general, smaller sized nanoparticles provide a higher surface area to volume ratio, faster reactivity and greater mobility than their larger counterparts. However, in some cases, the small size of nanoparticles allows for their enhanced penetration into biological and environmental substrates, which can sometimes lead to negative effects such as cell toxicity or environmental pollution. Thus, in some cases, larger sized nanoparticles are needed for achieving desired properties. Even a small change in size can result in a large difference in several important properties of nanoparticles. Therefore, the precise size of the nanoparticles on a continuous scale is needed for different applications.
The common methods of synthesizing silica nanoparticles are the Stöber method and the water-in-oil microemulsion method. Although these methods have led to production of nanoparticles of various sizes, the sizes are limited, discrete, and not precisely tunable on a continuous range.